I developed this a bit further, though there's still more I hope to do with it.
It turns out that building a custom runtime is discouraged; the better way to support game developers is to build a 'base app', which people can then add their own game files to. I have prepared two different base apps: one includes Python 3.6, and makes a download of about 30 MiB. The other uses Python 3.4 from the shared runtime, so is a download of about 7 MiB. My idea is that the game developer can choose between the latest language features and a quicker installation.
My next step is to make a more complete example of using this to package a game (so far, I've tested with the 'aliens' example that ships with pygame). I might try with the solarwolf example on Pygame's Github org - or if anyone wants to suggest another suitable open-source game based on pygame, I could try with that.